Preview unavailable in edit mode

No preview in edit mode. Instead the users see this text:

Preview Unavailable

Failed to load the preview within the given time constraints.

The server took to long to return the preview. The default preview timeout is 15 seconds.

Update the preview timeout by adding this configuration in web.config:

<appSettings>
    <!-- Set iframe timeout to 20 seconds -->
    <add key="episerver:ui:PreviewTimeout" value="20000" />
</appSettings>

The value is in milliseconds, increase if necessary.

<%= heading %>